1. Field of the Invention
This invention relates to a fuel supply system for use in an internal combustion engine, and more particularly, to a fuel supply system which may provide a mixture charge of increased air fuel ratio, as compared with the stoichiometric air fuel ratio, in case the engine remains in a low load running mode. In addition, the present invention is further associated with a fuel supply system which increases the air fuel ratio of a mixture charge, when cooling water temperature and the load of an engine exceed given levels, respectively, and slows down transient phase from the stoichiometric air fuel ratio to the increased air fuel ratio or vice versa.
2. Description of the Prior Art
SAE papers 730005 and 730566 teach that harmful constituents of exhaust gases may be removed by means of a three way catalytic converter which may convert into an innocuous form, the three harmful emissions, such as nitrogen oxides (NOx), unburnt hydrocarbon (HC) and carbon-monoxide (CO) at the same time, while maintaining the air fuel ratio of a mixture charge in the proximity of the stoichiometric air fuel ratio.
In addition, U.S. Pat. Nos. 3,738,341 and 3,759,232 disclose fuel supply systems which measure the air fuel ratio of a mixture charge, based on the oxygen concentration of exhaust gases, and produce a signal to thereby control, to a desired level, the air fuel ratio of the mixture charge to be supplied to the engine according to the aforesaid signal representing the air fuel ratio measured.
With the prior art internal combustion engines having the aforesaid three way catalytic converter, when an engine is in a low load running mode such as idling, engine braking or steady running mode, with a throttle value maintained almost closed, considerable amounts of CO and HC are produced according to the operational characteristics of the engine. In addition, since a small amount of fuel is supplied to the engine at the time of low load running thereof, the air fuel ratio tends to vary due to minor changes in the condition of the air which is being supplied to the engine or changes in other running conditions. Accordingly, the prior art internal combustion engines fail to remove CO and HC to the extent desired.
The three-way converter poses a problem in that the desired catalytic action may only be achieved for a mixture charge of an air fuel ratio in a quite narrow range around the stoichiometric air fuel ratio. Accordingly, the prior art fuel supply systems fail to provide a lean mixture charge in case the temperature of cooling water in the engine is above a given level and the load on the engine exceeds a given level.